The present invention relates generally to fabrication of objects and prototypes through the sequential deposition of material. More particularly, the invention relates to the consolidation of material increments using friction joining to form objects.
Numerous manufacturing technologies for producing objects by sequentially adding material exist, with the casting of liquid metal being perhaps the oldest such technique. In the past two decades, various processes for fabricating objects to net shape solely through material addition, i.e. without a finishing step such as machining to produce detailed, high-precision features, have -been patented and, in a few cases, commercialized.
Most of these additive manufacturing processes either rely on an adhesive, or a solidification process in order to produce a bond between previously deposited material and each incremental volume of material which is added. Although the use of adhesives is convenient, the properties of the adhesive control the properties of the finished object, and this limits the usefulness of such processes in the production of engineering parts and products.
Processes which use solidification transformations result in objects with relatively uniform physical and mechanical properties, because the liquid which is present as each volume of material is added wets the previously deposited material, effectively acting as an adhesive with properties identical to those of the bulk material.
The most commercially successful of these technologies is stereolithography, in which a focused light source (typically an ultraviolet laser) is used to solidify a liquid photocuring polymer. As the laser focal point travels through a vat of liquid polymer, the polymer locally solidifies, and eventually, through appropriate programming of the motion of the focal point, a solid object is built.
Although several techniques have been developed and commercialized, the technologies available for additively producing metal objects are limited. Since the Bronze Age, humans have used forging as a means of producing objects by adding small volumes of material to shapes and hammering them to final dimensions. More recently, three-dimensional arc welding (shape melting), as described and patented by Edmonds et al., (U.S. Pat. No. 4,775,092) has been suggested as an approach to production of net shape metal components.
Prinz, U.S. Pat. No. 5,207,371, has also developed shape deposition modeling in which two types of molten metal are sequentially deposited to produce net shape. Prinz and others have shown that in addition to arc welding, laser deposition and thermal spraying may be used as the basis for forming net shape objects layer by layer, if masks are used at intervals sufficient to define the cross sections of the desired object (See U.S. Pat. No. 5,126,529). Kovacevic has refined the methods of Edmonds and included milling to improve object dimensional accuracy.
Laser melting and deposition have been developed extensively in the U.S. and Germany. Based on cladding technologies developed in the 1980s, processes such as laser engineered net shaping and direct metal deposition are being commercialized (See Lewis, U.S. Pat. No. 5,961,862). Laser direct metal deposition is under development by researchers around the world, including Sandia National Laboratory, Los Alamos National Laboratory, Optomec Inc., and Precision Optical Manufacturing in the United States, and the Fraunhofer Institute in Germany. In essence, the process involves the injection of metal powders into a high power laser beam, while the laser is rastered across a part surface. The powders are melted in the beam, and deposited under the influence primarily of gravity.
Other processes for producing net shape metal objects via material deposition involve the use of low melting point materials to join sheets or powders. For example, brazing of laminated objects has been described (patents) in which steel sheets are cut to the geometries of sequential cross sections of a part, and then furnace brazed together. A copper, titanium or nickel based braze alloy is generally used, with copper alloys having the lowest melting points, and ease of use.
A closely related technique uses infiltration of a low surface tension, low melting point alloy to fill voids in object made by compacting or printing metal powders has also been employed. For example, Cima et al. have patented a three-dimensional printing process, in which metal powders are ink jet printed in layers, and a binder is used to hold the shape of the printed object (U.S. Pat. No. 5,387,380). Following firing of this green part to remove the binder, the infiltrant can be added to produce a solid metal object (Dillon Infiltrated Powdered Metal Composite Article (U.S. Pat. No. 4,327,156). This technique is being commercialized by Extrude Hone Corporation. Other powder metallurgy techniques for producing metal objects to net shape involve the use of a pattern against which powders are densified using various combinations of elevated temperatures and pressures to produce a fully dense, net shape part.
In U.S. Pat. No. 5,578,227, Rabinovich describes a method in which a wire or filament feedstock is used and applied to a growing object while maintaining a substantially identical cross section by remotely heating the nit point at which the feedstock is fed onto and is tangent to the existing surface. Rabinovich proposes use of a laser to heat this location to the melting point.
Electroforming, or plating, has also been commercialized for additive manufacturing of metal components. This mature technology has recently been used to produce shells on near net shape patterns to produce objects, usually tooling inserts for the injection molding process. Electroforming is a very slow process. It typically takes up to two weeks to produce a shell 0.25xe2x80x3 thick in a material such as nickel which has sufficient strength and wear resistance to be used as permanent tooling. As a result, this process is used only to create shells which require backfilling by some secondary material. Metal powder filled epoxies are most often used, however, ceramic slurries, other plastics, cements, and low melting points metals have all been used.
Electroforming has other drawbacks besides extremely low deposition rate as a near net shape forming technology. In the electroforming process, metal salts are dissolved in an aqueous solution. When an electrical current passes through this bath, metal is deposited on the negatively charged surface (in net shape electroforming applications such as tooling, this will be a model which is the inverse of the desired final shape). Aqueous solutions of metal salts are generally toxic, and sludges form in these baths as a byproduct of the process. Both the liquid and the sludges are hazardous materials which must be handled and disposed of appropriately. It is noteworthy that Andre has patented a method of fabricating layered structures using masks and electroplating (U.S. Pat. Nos. 5,976,339 and 5,614,075).
More recently, nickel vapor deposition has been employed as a means of producing nickel shells for net shape fabrication applications. Nickel vapor deposition (NVD) allows thicker shells to be produced as deposition rates are higher than electroforming (Milinkovic, 1995). However, NVD involves the use of highly toxic gases and a specialized reaction chamber. The cost and risk of this technology are both very high.
The joining and cladding of metals using friction is a well known technology (Welding Handbook, Vol. 2). Friction processes include inertial welding, linear friction welding, friction surfacing, friction acoustic bonding and friction stir welding. Friction joining processes create heat at the faying surfaces of the materials to be joined, by rubbing the surfaces together. In addition to creating heat, this rubbing removes contaminants and oxide layers creating localized, atomically clean joint surfaces. It is known that the nature of metallic atomic bonds is such that if a very clean surface can be generated and maintained, joints between metals can be formed at relatively low temperature and pressure. Such solid-state consolidation could address many of the shortcomings of net-shape fabrication processes relying on liquid-to-solid transformation.
Friction bonding has been applied previously to the cladding and surfacing of objects for corrosion protection or repair of worn areas. Reference is made to U.S. Pat. No. 5,469,617, 5,183,390, 5,077,081, 4,959,241 and 4,930,675, which disclose various surfacing methods based upon friction bonding. So far, however, the technique has not been applied to object formation.
This invention resides in the use of friction heating and bonding to consolidate sequentially applied metals, plastics or composites to previously deposited material so as to form a bulk deposit in a desired shape. Monolithic or composite sheets, tapes and filaments may be consolidated using the approach.
In terms of apparatus, a system according to the invention includes a source of friction; a mechanism for applying a forging load between a feedstock power supply and a work surface; a work-head, which may have various configurations depending on the geometry of the feedstock to be used; a material feeding system; and a computer-controlled actuation system which controls the placement of material increments added to an object being built. A computer model of the object to be built is used to generate commands to produce the object additively and automatically.
The technology represents a dramatic improvement over processes now in use. The inventive approach provides a solid, freeform fabrication technique that requires no tooling, operates in the solid state, and creates a bond directly at the faying surfaces (i.e., acts only at the location where bonding/ consolidation of the material increments is desired). It also addresses the problem of hazardous materials and energy sources, and also the engineering problems inherent in the handling of liquid metals.